Interest in catalysis continues to grow in the polyolefin industry. There are several known processes to polymerize olefins including solution, slurry, and gas phase processes. For some processes, it is preferred to use a solid catalyst and one common strategy is to support a transition metal compound on magnesium chloride. The surface area affects the availability of the transition metal compound and therefore the activity of the catalyst. The greater the activity of the catalyst, the less the residual transition metal which is left in the polyolefin. High levels of residual transition metal can have adverse effects on such properties as color and thermal stability.
There have been several approaches to obtaining magnesium chloride with high surface area. For example, U.S. Pat. No. 4,421,674 discloses a spray drying method and compares it with several other methods. They are able to prepare small particle size magnesium chloride by heating a concentrated solution of magnesium chloride in ethanol and spray drying the solution with a stream of hot nitrogen. Spray drying has advantages versus other techniques that can give low particle size such as milling. Unfortunately, milling yields irregular particles. When these particles are treated with titanium tetrachloride, the catalysts have lower activity than titanium tetrachloride supported on spray-dried magnesium chloride.
The '674 patent also teaches to dissolve commercially available flake magnesium chloride with an average particle size of from 0.1 to 2 mm in anhydrous ethanol and heat to concentrate the solution. The magnesium chloride precipitates as elongated needles with an average size of 30 microns. This is treated with titanium tetrachloride, but again gives poorer results than those obtained by using spray-dried magnesium chloride. Spray drying requires extra equipment and it would be desirable to have a simpler process.
Further, as noted in U.S. Pat. No. 5,173,465, there are difficulties and complications inherent in the spray drying operation. They attempt to overcome these difficulties by impregnating porous silica having a mean particle size of between 10 and 100 microns. The silica is slurried in an ethanol solution of magnesium chloride to impregnate the porous silica and then the slurry is heated to remove the ethanol to a level of between 1.5 to 4 moles ethanol per mole magnesium chloride. The silica represents 50-90% by weight of the solid catalyst component with the remaining 50-10% containing titanium, magnesium, chlorine, and alkoxy groups. Even at 50%, this doubles the weight of solid that must be added to the polymerization.
It would be desirable to have a particle of magnesium chloride that can be used as a support for transition metal compounds to form highly active catalysts without the difficulties associated with spray drying and without using high levels of silica having a large particle size.
There has been considerable work with other systems regarding crystal size. Much of the work involves ensuring a large uniform crystal. One technique used to influence crystal size is to seed a supersaturated solution. The crystallization of potassium alum, KAI(SO4)2.12H2O, has been studied extensively by Kubota et al, as reported in Powder Technology 121 (2001) 31. They state that while “seeding has been known for a long time as an effective technique . . . . No one can predict what amount of seeds and what size of seeds should be added into a crystallizer to produce a product of desired size, or to stabilize crystallization. No methodology has been proposed. Seeding seems to be treated as an art.”
In the study of potassium alum, Kubota made two key findings. There is a critical seed concentration. Above that seed concentration, the crystals of potassium alum were unimodal in size distribution and bimodal below that concentration. Also, the mean size of the potassium alum crystals increased linearly with the mean mass size of the seeds. The sizes of the seeds studied were from 40 microns to 300 microns.
Magnesium chloride has been used for more than 20 years and the concept of seeding to influence crystallization has been known for more than 100 years. However, there remains a need to make magnesium chloride of uniform and small particle size without the complications of spray drying.